Three-dimensional morphology of ferrite formed in association with inclusions in low-carbon steel
Introduction
A good combination of strength and toughness of low-carbon steel welds is achieved by so-called acicular ferrite microstructure, consisting of small interweaving ferrite plates formed within austenite grains [1], [2], [3], [4], [5], [6]. The term acicular is used frequently in welds where ferrite plates are relatively small and cover a large proportion of the matrix. The interweaving microstructure is attributed to nucleation of multivariant ferrite plates at intragranular inclusions, growth thereof, and nucleation of secondary plates at the surface of preexisting ferrite plates [2], [6], [7]. Since ferrite transformation occurs fast and the microstructure is often observed after the weld is cooled to ambient temperature, studies of morphology and growth behavior of ferrite crystals at early stages are relatively scarce in low-carbon steel.
In recent years, both computer hardware and software for image processing and three-dimensional (3D) visualization have developed rapidly. These developments prompted the application of 3D reconstruction to opaque metallic microstructures for the past decade, as reviewed by Kral et al. [8]. The characterization of microstructure in three-dimensions often plays an essential role in identification of transformation mechanism [8], [9], [10], [11], [12], [13], [14] and quantitative analysis of microstructure [15]. In this report, low-carbon steel specimens were reacted for very short times to obtain partially transformed microstructure and the microstructure was 3D-reconstructed to observe the morphology of ferrite plates formed within the austenite matrix. Then, the length, width, and thickness of individual ferrite crystals were measured from 3D-reconstructed images and the growth behavior of ferrite plates was discussed.
Section snippets
Experimental procedure
Table 1 shows the compositions of the base plate (SM490), low-carbon weld wire and specimens taken from the weld deposit. The welding was conducted under a shielding gas of 100% CO2. It is seen that the concentrations of carbon and substitutional elements were all reduced after welding, while the oxygen concentration is large. The specimens were first austenitized at 1200 °C for 20 min and furnace-cooled to obtain a grain size as large as 80 μm. They were subsequently austenitized at 1350 °C
Three-dimensional image of ferrite plate
Fig. 1a and b shows the light optical micrographs of specimens reacted at 570 °C for 1 and 5 s, respectively. It is seen in Fig. 1a that thin long plates are nucleated in the matrix, presumably at oxide inclusions. At this stage, the appearance of ferrite plates is very similar to intragranular ferrite plate in the earlier classification system of ferrite morphology [16]. In Fig. 1b, the number of ferrite plates increased and plates are considerably thickened. The microstructure evolved to a
Summary
The 3D morphology and growth behavior of acicular ferrite at early growth stages in low-carbon steel were studied by serial sectioning and computer-aided 3D reconstruction. The specimen taken from the weld deposit was austenitized and isothermally reacted at 570 °C for 1 and 5 s. Ferrite plates were observed to be nucleated at intragranular inclusions, presumably on oxides, and grew radially along specific directions in the matrix austenite, thus forming an arrangement of ferrite crystals noted
Acknowledgements
This work was conducted as a part of the research activities of “Development of Highly Efficient and Reliable Welding Technology” project under the auspices of the Japan Space Utilization Promotion Center (JSUP, Tokyo). The authors express thanks to Materials Research Lab, Kobe Steel, for conducting welding and heat treatment of specimens.
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Present address: Department of Applied Physics, Wuhan University of Science and Technology, Wuhan 430081, PR China.
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Present address: Dietec Inc., Mibu-cho, Shimotuga-yun, Tochigi 321-0215, Japan.